If you are already using packages and blueprints for your application (Modular Applications with Blueprints) there are a couple of really nice ways to further improve the experience. A common pattern is creating the application object when the blueprint is imported. But if you move the creation of this object into a function, you can then create multiple instances of this app later.
So why would you want to do this?
So how would you then actually implement that?
The idea is to set up the application in a function. Like this:
def create_app(config_filename): app = Flask(__name__) app.config.from_pyfile(config_filename) from yourapplication.model import db db.init_app(app) from yourapplication.views.admin import admin from yourapplication.views.frontend import frontend app.register_blueprint(admin) app.register_blueprint(frontend) return app
The downside is that you cannot use the application object in the blueprints at import time. You can however use it from within a request. How do you get access to the application with the config? Use
from flask import current_app, Blueprint, render_template admin = Blueprint('admin', __name__, url_prefix='/admin') @admin.route('/') def index(): return render_template(current_app.config['INDEX_TEMPLATE'])
Here we look up the name of a template in the config.
It’s preferable to create your extensions and app factories so that the extension object does not initially get bound to the application.
Using Flask-SQLAlchemy, as an example, you should not do something along those lines:
def create_app(config_filename): app = Flask(__name__) app.config.from_pyfile(config_filename) db = SQLAlchemy(app)
But, rather, in model.py (or equivalent):
db = SQLAlchemy()
and in your application.py (or equivalent):
def create_app(config_filename): app = Flask(__name__) app.config.from_pyfile(config_filename) from yourapplication.model import db db.init_app(app)
Using this design pattern, no application-specific state is stored on the extension object, so one extension object can be used for multiple apps. For more information about the design of extensions refer to Flask Extension Development.
So to use such an application you then have to create the application first in a separate file otherwise the flask command won’t be able to find it. Here an example
exampleapp.py file that creates such an application:
from yourapplication import create_app app = create_app('/path/to/config.cfg')
It can then be used with the flask command:
export FLASK_APP=exampleapp flask run
The factory function from above is not very clever so far, you can improve it. The following changes are straightforward and possible:
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